Generally, discrete fixed radio frequency (RF) filters are costly, large, and cover a narrow frequency range. However, in some radio technologies, such as in Long Term Evolution (LTE), the RF front-end may include a plurality of bands, such as over 29 frequency division duplexing (FDD) bands and over 11 time division duplexing (TDD) bands as defined by for example the Third Generation Partnership Project (3GPP). As such, tunable filters, rather than discrete fixed filters, may provide a way to cover these bands. Moreover, the tunable filter may implement a phase shifter to tune over the wide range of frequencies associated with these bands.
Phase shifters based on capacitively loaded transmission lines may be narrow band. As such, when the phase shift is increased, the characteristic impedance of the capacitively loaded transmission lines may decrease. This type of phase shifter may be difficult to interface to bandpass resonators because these bandpass resonators may be predefined for a specific fractional bandwidth and center frequency. Alternatively or additionally, switched-line phase shifters may be used in a tunable filter, but switched-line phase shifter may be limited by the number of independent transmission lines needed to support the combination of coarse phase shift (Phi) and coarse characteristic impedance (Zo)—limiting thus the phase shifter's frequency of operation.
FIG. 16 depicts a typical switched-line phase shifter configured to selects one of two transmission lines to perform coarse phase-shifting. This type of phase shifter may use a transmission line for independent coarse phase shift (Phi) and a transmission line for coarse characteristic impedance (Zo). As a result, supporting multiple Zo and Phi states may require the use of bulky, higher-order multi-throw switch matrices, such as the single pole, four throw switch (labeled S1 and S2) depicted at FIG. 16, and may require an area intensive set of transmission lines.